DENSITY-DEPENDENT FORAGING IN THE SEA-URCHIN CENTROSTEPHANUS-RODGERSII ON SHALLOW SUBTIDAL REEFS IN NEW-SOUTH-WALES, AUSTRALIA

An experimental manipulation of density in the diadematid sea urchin Centrostephanus rodgersii demonstrated that the species has a large impact on the abundance of large brown algae, limpets and crustose coralline algae. Significant non-linearities were observed in these variables in response to the removal of all, 66 %, 33 % or no sea urchins from naturally occurring patches of Barrens habitat in New South Wales. Removal of all sea urchins caused the loss of the Barrens habitat and the development of an assemblage of foliose algae. Foliose algae did not successfully colonize treatments in which only some sea urchins were removed. Partial removals caused reductions in the size of patches that were not linearly related to density. Changes in size of patches were interpreted as being a complex product of the size and shape of crevices containing sea urchins.